No abstract

We have measured the thermochromism of pyridinium‐N‐phenoxide betaine and its penta‐tert‐butyl substituted derivative dissolved in MTHF together with the solvent dielectric properties in order to relate polarity scales to the dielectric constants of the solvent. Both thermochromic and solvatochromic shifts of the absorption band of the dyes can be rationalized on the basis of the MSA theory for relating the solvation free energy to the static dielectric constant. Deviations between theory and experiment in the regime of higher viscosities can be attributed to changes in the offset between absorption maximum and electronic transition energy.

No abstract

The orientational dynamics of organic supercooled liquids of low molecular weight confined to the geometry of porous glasses are studied by two highly related techniques, the optical method of probing the dynamics of solvation regarding a chromophoric host molecule and dielectric relaxation spectroscopy. The dielectric results display marked effects of the confinement to mesopores in terms of altered structural dynamics which appear to separate into a raster and slower responses relative to the bulk liquid. We also demonstrate that there is no trivial relation between the ε*(ω) data and the liquid dynamics in these heterogeneous samples. These effects are partially paralleled by the solvation dynamics results, but with the spatial range inherent in the optical technique being inconsistent with associating the fast and slow dynamical components to spatially distinct regimes. We conclude on the slow component being a signature of non-ergodicity which arises from the competition between the length scale of cooperativity and the pore size.

We combine the results of dielectric relaxation spectroscopy and solvation dynamics measurements for the glassforming liquid 2-methyltetrahydrofuran spatially confined to the pores of sol-gel glasses with pore diameters @ = 2.5, 5.0, and 7.5 nm. Although both techniques probe the orientational polarization of the polar liquid, a dielectric experiment is bound to sense macroscopic effects, whereas the solvation method probes the molecular dynamics on microscopic scales. When the tmeperature approaches the glass-transition at Tg upon cooling, an interfacial liquid layer with strongly frustrated dynamics grows in thickness and is thus able to block the connectivity among the pores. As a result the structural a-process is subject to an enhanced spatial restriction near Tg and a transition from constant pressure to constant volume conditions might occur as the pore connectivity becomes obstructed by immobile liquid.